Double-hop transmission is a condition where two ground stations may indirectly communicate with each other with the assistance of an intermediate ground station. A double-hop satellite link typically arises where two ground stations are sufficiently remote from each other that they do not have a direct link to the same satellite. As a result, the signal travels through space twice.
In particular, a transmitting ground station sends the signal through a first satellite to the intermediate ground station. Then, the intermediate ground station sends the signal through a second satellite to a recipient ground station.
Several disadvantages of double-hop satellite links are known. Double-hop satellite links double the satellite delay normally attributed to direct satellite links. Also, double-hop transmission is known for producing an inconvenient delay in sound for video conferencing. In this regard, a user may see parts of a video transmission before the associated sound is played. Furthermore, service costs may be doubled and security problems may arise with the use of an additional satellite and an additional ground station.
One way to reduce double hop delays is to provide an inter-satellite link to allow for secure, high capacity communication channels around the world. One problem with inter-satellite links is that the transmitter power is relatively high. In satellite communications, reducing the power consumption by the various components is typically a goal. One way in which to reduce output power by fifty percent is to employ an encoding scheme known as differential phase shift keying (DPSK). Differential phase shift keying requires a demodulator on the receiver to translate the received data from optical phase encoding to standard logical zeroes and ones. In DPSK, each bit is compared with the previous bit and, if the bit differs from the previous, a phase change of 180 degrees is placed on the optical carrier. If the bit is the same, no change in phase is made. DPSK has an advantage over regular phase shift keying in that there is no need for a local oscillator or phase reference. An optical system, employing a local oscillator as a reference, is a complex, non-trivial problem that is extremely difficult to implement.
Data encoding using DPSK uses an optical interferometer. The optical interferometer is sensitive to length changes on the order of tens of nanometers. Thus, a large degree of active length stabilization is required. However, as mentioned above, it is also desirable to minimize the amount of power consumption employed in any compensation scheme.
It is therefore desirable to provide a compensation scheme for an interferometer that minimizes the amount of power consumption by the satellite.